Minkowski’s Proper Time and the Status of the Clock Hypothesis1 1 I am grateful to Stephen Lyle, Vesselin Petkov, and Graham Nerlich for their comments on the penultimate draft; any remaining infelicities or confusions are mine alone. Minkowski’s Proper Time and the Clock Hypothesis Discussion of Hermann Minkowski’s mathematical reformulation of Einstein’s Special Theory of Relativity as a four-dimensional theory usually centres on the ontology of spacetime as a whole, on whether his hypothesis of the absolute world is an original contribution showing that spacetime is the fundamental entity, or whether his whole reformulation is a mere mathematical compendium loquendi. I shall not be adding to that debate here. Instead what I wish to contend is that Minkowski’s most profound and original contribution in his classic paper of 100 years ago lies in his introduction or discovery of the notion of proper time.2 This, I argue, is a physical quantity that neither Einstein nor anyone else before him had anticipated, and whose significance and novelty, extending beyond the confines of the special theory, has become appreciated only gradually and incompletely. A sign of this is the persistence of several confusions surrounding the concept, especially in matters relating to acceleration. In this paper I attempt to untangle these confusions and clarify the importance of Minkowski’s profound contribution to the ontology of modern physics. I shall be looking at three such matters in this paper: 1. The conflation of proper time with the time co-ordinate as measured in a system’s own rest frame (proper frame), and the analogy with proper length. 2. Misconceptions that Special Relativity (SR) applies only to objects in inertial motion, and not to accelerated systems, and that therefore one must introduce General Relativity to solve Langevin’s Twin Paradox. 3. Misconceptions surrounding the status of the so-called clock hypothesis (CH), according to which the instantaneous rate of a clock depends only on its instantaneous speed, and not on its acceleration. I shall argue that the CH is a criterion for ideal clocks that is implicit in SR, and does not have the status of an independent assumption; and that it also performs this role in GR as a consequence of the strong equivalence principle. 1. Proper Time, Local Time and Proper Length The first symptom of this under-appreciation of the novelty of proper time I wish to discuss is that it is often taken to be the time co-ordinate measured by a clock at rest in an inertial frame: in its own frame, therefore, proper, as opposed to local time. ‘Local time’, of course, was originally Lorentz’s term for the transformed co-ordinate time t´ of an inertial frame of reference moving with velocity v with respect to the stationary frame. By Lorentz’s own admission, the chief cause of his failure to discover Special 2 Here I concur with Roberto Torretti, who rates Minkowski’s introduction of proper time as “probably his most important contribution to physics” (1983, p. 96). 2 Minkowski’s Proper Time and the Clock Hypothesis Relativity “was my clinging to the idea that only the variable t can be considered as the true time, and that my local time t´ must be regarded as no more than an auxiliary mathematical quantity”.3 Einstein’s correction lay in seeing that all of these time co-ordinates or local times are on a par and equally entitled to be regarded as the true time. It is perhaps for these reasons that the illusion arose that proper time is simply the local time of the system’s own rest frame, the time co-ordinate as measured in an inertial frame in which the system is at rest. And this in turn could explain why it is not uncommon to find the introduction of proper time attributed to Einstein, even by authors with a keen sense of the history of physics.4 The conflation of proper time with co-ordinate time in a system’s own rest frame is also perhaps fostered by the numerical equivalence of the value of the proper time elapsed for a body moving along an inertial path with the value measured by the time-co-ordinate in its rest frame. Thus it is often said that proper time is simply time measured in a body’s “proper frame”, as if a body keeps its own inertial frame while accelerating! There are two confusions here: first, the idea that a body “has” an inertial frame, when a reference frame is just a point of view for representing the body’s motion, and (according to the principle of relativity) one can represent this motion equivalently from any inertial frame; and second, of course, the idea that the body could stay in the same inertial frame even though it is accelerating, and therefore moving non-inertially. At any rate, this is a confused idea of proper time, which is not a time co-ordinate and was not introduced by Einstein, but by Minkowski, in his famous paper of 1908 (Lorentz et al. 1923, 73-91). Introducing the concept, he asks us to imagine at any point P (x, y, z, t) in spacetime a worldline running through that point, so that the magnitude corresponding to the timelike vector dx, dy, dx, dt laid off along the line is dτ = √(c2dt2 – dx2 – dy2 – dz2)/c (1) Proper time is now defined as the integral of this quantity along the world line in question: “The integral τ = ∫ dτ of this quantity, taken along the worldline from any fixed starting point P0 to the variable endpoint P, we call the proper time of the substantial point at P.” (85) Thus for Minkowski spacetime, the proper time is: 2 2 2 2 2 τ = ∫P dτ = ∫P √(c dt – dx – dy – dz )/c (2) or, equivalently, 3 Quoted from (Torretti 1983, p. ). 4 See for example J.-P. Provost, C. Bracco and B. Raffaelli 2007, p. 498: “If one starts with SR, …the first important notion, as Einstein told us in 1905, is the proper time τ”. 3 Minkowski’s Proper Time and the Clock Hypothesis P 2 2 2 2 1/2 τ = ∫P0 {1 – 1/c [(dx/dt) + (dy/dt) + (dz/dt) ]} dt (3) As defined, the proper time cannot be evaluated without adopting a system of co-ordinates. But because of the signature of the Minkowski metric, the interval dτ and its integral τ are both invariants, so that their values are independent of what system of co-ordinates is adopted. Thus for the time elapsed along any worldline, τ gives a measure that is independent of the co-ordinates, even if a particular frame must be adopted in order to calculate its value. The whole content of relativity theory can now be framed in terms of such invariants, so that co-ordinates are no longer regarded as primitive, as they had been in Einstein’s way of conceiving SR. For instance, as Minkowski proceeded to explain, x, y, z and t —the components of the vector OP, where O is the origin— are considered as functions of the proper time τ, and the first derivative of the components of this vector with respect to the proper time, dx/dτ, dy/dτ, dz/dτ and dt/dτ, are those of the velocity vector at P, which is also a four-dimensional invariant. As is well known, the resulting four-dimensional co-ordinate-free rendering of special relativity is of immense utility for the further development of relativity theory, even if Einstein did not at first appreciate its significance. The misidentification of proper time as the time co-ordinate in its rest frame is also encouraged by an analogy with proper length, which is the length of a body in its rest frame. The analogy, it is often claimed, is perfect, and the invariance of proper time is no objection. For just as the length of a path joining two events in timelike separation is invariant under change of frame, so is the length of a curve joining two events in spacelike separation.5 This can be seen by comparing the expressions for proper time and proper length written in the tensor form necessary for general relativistic spacetimes. Here the appropriate generalization of the expression given by Minkowski for his flat spacetime to curved spacetimes is6 α β 1/2 τ = ∫P dτ = ∫P (–gαβ dx dx ) (4) The flat-space expression for the proper length should be generalized so that it is the exact analogue of proper time, a line integral along a curve joining two spacelike separated events: 5 Cf. the article on proper length in Wikipedia (http://en.wikipedia.org/wiki/Proper_length: March 24, 2009). The author suggests a generalization of proper length so that (in either Special or General Relativity) it is given by the µ ν line integral L = c ∫P √[–gµν dx dx ], where gµν is the metric tensor for the spacetime with +--- signature, normalized to return a time, and P is the spacelike path. The author also notes that “Proper length has also been used in a more restricted sense to help with discussions of length contraction by textbooks, where it is defined as the length of an object when measured by someone at rest relative to that object.” 6 Cf. Misner, Thorne and Wheeler, 1973, 393. 4 Minkowski’s Proper Time and the Clock Hypothesis α β 1/2 L = ∫P ds = c ∫P (gαβ dx dx ) (5) There is no doubt that this defines a proper distance, that between the endpoints of a path in spacetime along which no process can travel, a spacelike curve.